The present invention relates to an optical recording medium for recording and reproduction using a floating type optical head, and a process for producing the same. More particularly, it relates to an optical recording medium which is enhanced in seeking resistance while the floating type optical head seeks through the optical recording medium, and is inhibited or prevented from a lubricant of the optical recording medium from sticking to the slider surface of the floating type optical head and to the light emitting face of a solid immersion lens, and to a process for producing the optical recording medium.
With coming of the multimedia age, optical recording media are widely utilized as media capable of recording and reproducing a large quantity of information such as music, animation images, and data of computers. Optical recording media are classified into read-only type optical recording media capable of only reading of information, such as CD and laser disks, write-once type optical recording media capable of writing only one time, such as CD-R, and rewritable optical recording media capable of rewriting and erasing any number of times.
As one of the rewritable optical recording media, for example, magneto-optical recording media are known. Magneto-optical recording media usually have a structure comprising a transparent substrate and, laminated thereon in succession, a first dielectric layer, a magneto-optical recording layer, a second dielectric layer, a reflective layer and a protective layer, and recording and reproduction are carried out using an optical head and a coil for generation of magnetic field.
Recently, with increase of quantity of information to be handled, further increase of storage capacity is demanded for optical recording media. For meeting the demand, a method is proposed which performs recording and reproduction using a floating type head on which a solid immersion lens (SIL) is mounted. According to this method, recording and reproduction can be performed with a smaller light spot diameter than according to conventional methods, and, hence, recording density per unit area can be increased by about one figure as compares with conventional magneto-optical recording media.
When recording and reproduction are carried out using SIL, a proximate light which oozes from the SIL can be used as a light for recording and reproduction. Since the proximate light decays at a distance of about xc2xc the wavelength of the light, the distance between the SIL and the recording layer must be less than xc2xc the wavelength of the recording and reproducing light. For example, when a red laser beam of 680 nm in wavelength is used, the distance between the light emitting face of the solid immersion lens and the recording layer must be 170 nm or less. Since thickness of the transparent substrate in conventional magneto-optical recording media is about 1.2 mm, it is difficult for the proximate light to reach the recording layer through the substrate. Therefore, in case a proximate light is used, light is projected into the recording media from the side opposite to the substrate. For this reason, the sequence of lamination for making magneto-optical recording media using a proximate light differs from that of conventional media and usually the former media have a structure comprising a transparent substrate and, laminated thereon in succession, a reflective layer, a first dielectric layer, a magneto-optical recording layer and a second dielectric layer. Furthermore, recording and reproduction are carried out using a floating type optical head, and, therefore, the floating type optical head contacts with the medium when the optical head conducts seeking operation. Therefore, a protective layer and a lubricating layer are formed in succession on the second dielectric layer of the medium in order to enhance seeking resistance.
The present inventors disclose, in JP-A-11-7657, an optical recording medium comprising a substrate and, provided thereon, a reflective layer, a recording layer and a dielectric layer and irradiated with a recording light or reproducing light from the side of the dielectric layer, in which a protective layer having self-lubricating property is provided on the dielectric layer and a lubricating layer is further provided on the protective layer. As compounds constituting the lubricating layer, there are disclosed perfluoro polyethers having one hydroxyl group, carboxyl group, ester group or piperonyl group at least one end of the molecule because of their large adsorption energy to the protective layer comprising carbon.
However, when a floating type optical head seeks through the magneto-optical recording medium, there is the possibility of a part of the lubricant being vaporized by the heat of laser beams to move and stick to the light emitting face of SIL or the SIL sliding over the lubricant to cause moving and sticking of the lubricant to the light emitting face of the SIL. If the lubricant moves and sticks to the light emitting face of SIL, optical characteristics of SIL are changed and recording and reproduction cannot be satisfactorily performed. As the causes therefor, the following can be considered. That is, lubricant for optical recording media according to optical recording system using SIL has a high transparency, being different from the lubricant for magnetic recording media according to magnetic recording system such as magnetic tape and magnetic disk, but the lubricant which moves and sticks to the light emitting face of SIL is decomposed with heat of laser beams and changes in its properties to deteriorate the transparency. Thus, it is desired to coat the protective layer with a lubricant which is further improved in adsorption property to the protective layer than the lubricant disclosed in JP-A-11-7657.
The present invention has solved the above problems, and the object is to provide an optical recording medium which is inhibited or prevented from moving and sticking of lubricant to the light emitting face of an optical element such as SIL and can realize recording and reproduction using SIL.
Another object of the present invention is to provide an optical recording medium in which the lubricating layer comprises a lubricant which is not heat decomposed even if heated by irradiation with light beams and maintains a high transparency.
Still another object of the present invention is to provide a process for producing an optical recording medium in which the lubricant hardly moves and sticks to a light emitting face of optical elements such as SIL and which has a lubricating layer high in sliding resistance.
According to the first embodiment of the present invention, there is provided an optical recording medium comprising a substrate and, provided thereon in succession, a reflective layer, a recording layer, a protective layer and a lubricating layer comprising a lubricant, characterized in that a lubricant bonding rate of the lubricant to the protective layer is 60-100%.
One example of the structure of the optical recording medium according to the present invention is shown in FIG. 1. The optical recording medium 10 comprises a substrate 1 and, provided thereon in succession, a reflective layer 2, a first dielectric layer 3, a recording layer 4, a second dielectric layer 5, a protective layer 6 and a lubricating layer 7. As shown in FIG. 1, the lubrication layer 7 comprises a bonding lubricating layer 8 present on the protective layer 6 with bonding to the protective layer and a physical adsorptive lubricating layer 9 present on the bonding lubricating layer 8 and not bonded to the protective layer. As for the lubricating layer, it is known that this layer is also formed on the uppermost layer of conventional magnetic disks in order to reduce wear of the magnetic disk caused by magnetic head. The lubricant bonding rate is known as a measure for expressing the degree of bonding of such lubricating layer to a surface on which the lubricating layer is coated (for example, a protective layer). The lubricant bonding rate is expressed by the proportion of the bonding lubricating layer in the lubricating layer. In this specification, the lubricant bonding rate is numerically expressed in the following manner. Thickness of the lubricating layer is referred to as A, and thickness of the lubricating layer after the lubricating layer and the substrate including the coated surface or the information recording medium has been subjected to ultrasonic cleaning for 30 seconds in a solvent is referred to as B. In this case, the lubricant bonding rate is expressed by B/Axc3x97100%. According to the inventors"" investigation, it is found that the lubricant bonding rate of a magnetic disk is usually 30-50%.
The optical recording medium according to the first embodiment of the present invention is characterized in that the lubricant bonding rate of the lubrication layer is increased to 60% or more, considering the theory of recording and reproducing using the optical recording medium by the optical head. That is, since the lubricant bonding rate is 60% or more, amount of physical adsorptive lubricant on the bonding lubricant is small. Therefore, even in the case of optical recording and reproducing system by which the recording and reproduction are carried out with the optical head and the recording surface being in close vicinity to each other, for example, with the distance therebetween being 100 nm or less, such as the system of using a floating type optical head with an SIL mounted thereon, the physical adsorptive lubricant can be prevented or inhibited from moving and sticking to the light emitting face of the SIL. Furthermore, when the floating type optical head mounting an SIL seeks through the optical recording medium, the floating type optical head does not directly contact with the protective layer due to the presence of the bonded lubricant component, and thus the endurance of the optical recording medium can be further enhanced.
It is preferred that the lubricating layer comprises a perfluoro polyether lubricant having alcohol group, carboxyl group, ester group or piperonyl group at one or both ends of the molecule and having a molecular weight of 2000-10000. If the molecular weight is less than 2000, lubrication effect is insufficient and, besides, the lubricant is apt to be vaporized with increase of temperature. If the molecular weight exceeds 10000, viscosity of the lubricant increases to increase frictional coefficient and it becomes difficult to satisfactorily slide the optical head over the optical recording medium. Furthermore, perfluoro polyethers of 2000-10000 in molecular weight have a heat decomposition temperature of about 350xc2x0 C., and shows no absorption of light in visible light region when used in optical recording systems. Therefore, the lubricants can fulfill the functions as lubricants without deterioration of optical properties in optical systems.
In addition to the above lubricants, perfluoro polyethers in which one end group or both end groups of the molecule have two or more hydroxyl groups can be used as lubricants. These perfluoro polyether lubricants preferably have a molecular weight of 2000-10000. These lubricants include, for example, perfluoro polyethers represented by HOxe2x80x94CH2CH(OH)CH2OCH2CF2Oxe2x80x94(C2F4O)nxe2x80x94(CF2O)mxe2x80x94CF2CH2xe2x80x94CH(OH)CH2OH. In this formula, n and m are optionally selected so that the molecular weight of the perfluoro polyethers can be within the range of 2000-10000, and, for example, n and m may be nearly equal.
In order to further increase the endurance by bonding the lubricant to the protective layer, it is preferred that after the lubricating layer is formed on the protective layer, this is heated at a temperature in the range of 50-120xc2x0 C. or is irradiated with ultraviolet rays of 185-254 nm in wavelength, for example, ultraviolet rays of 185 nm and/or 254 nm which are usable at present, in the air or in an inert atmosphere. The bonding of the lubricant to the protective layer can be further accelerated by carrying out the heating and the irradiation with ultraviolet rays in combination.
In order to inhibit moving and sticking of the lubricant to the solid immersion lens as much as possible, the lubricant bonding rate is preferably 80% or more. For increasing the lubricant bonding rate to 80% or more, for example, the lubricating layer is formed on the protective layer and thereafter they can be rinsed with a solvent in addition to the heating and the irradiation with ultraviolet rays.
According to the second embodiment of the present invention, there is provided an optical recording medium comprising a substrate and, provided thereon in succession, a reflective layer, a recording layer, a protective layer and a lubricating layer comprising a lubricant, characterized in that the lubricant is a perfluoro polyether in which at least one end group of the molecule has at least two hydroxyl groups.
The optical recording medium according to the second embodiment of the present invention is characterized in that a perfluoro polyether in which one or both end groups of the molecule have two or more hydroxyl groups is used as a lubricant molecule, considering the theory of recording and reproducing using the optical recording medium by a floating type optical head. The perfluoro polyethers having two or more hydroxyl groups in one or both end groups of the lubricant molecule are twice or more in the number of functional groups as compared with those having one hydroxyl group in one or both end groups of the molecule disclosed in JP 11-7657 A, and, hence, are increased about twice or more in adsorptivity to the carbon protective film used as the protective layer. Thanks to this increase of adsorptivity of the lubricant molecule, sliding resistance increases, and frictional coefficient of the medium surface can be lowered and damaging of the medium surface (scratches by sliding) in seeking can be inhibited.